Signal converters of the kind indicated above are known where the current which is to be converted flows through a coil suspended in a magnetic field caused by a permanent magnet. The forces developed by the current and the magnetic field tend to push the coil out of the magnetic field. The force is compared, by a balance beam which is movable in a spring band cross, with the force which is developed in a bellows connected to the balance beam. If these forces are not alike, the position of the beam is altered. The new position is converted in a nozzle into a pneumatic pressure which acts on an upper diaphragm in a pneumatic amplifier belonging to the device. The device also includes a number of further components and functions which will not be described in more detail here.
This known device affords satisfactory linearity but also has certain decided disadvantages and limitations. Thus the high degree of complication of the device means that it is expensive which limits its field of application for economic reasons. The multiplicity of components also makes the device very demanding in space. This makes it difficult or impossible to use it in a limited space, as, for example, in connection with compact computerized centres. The number of movable components, particularly the comparatively long balance beam, also make it difficult to balance out which reduces the speed of reaction, that is to say the capacity to alter the pneumatic pressure signals quickly after alterations in the current signals. This is, in fact, a factor which is acquiring increasing importance in certain branches of industry, for example in the petrochemical industry (PE manufacture) and the paper industry (machine speeds up to 1200 m/min). The number of mechanical components in conventional systems also leads to friction which produces a delayed action which is expressed in the same manner as magnetic hysteresis, which limits the possibilities of achieving the best transfer characteristics. In addition, conventional systems are very sensitive to vibration which in many industrial connections is a very serious, not to say decisive disadvantage.
A further limitation of conventional electro-pneumatic signal converters is that they cannot use a working pressure of 4-6 bar, which is the prevalent working pressure in industry, without converters.